This invention relates to a pneumatic tire for heavy duty trucks.
Generally pneumatic tires for heavy duty trucks, as means of compatibly fulfilling two contradicting properties, one of ensuring traction and the other of warranting wear life, have main grooves (rib grooves M, lug grooves 3, etc.) and auxiliary grooves 4 of a smaller width than the main grooves disposed in a tread 1 as shown in FIG. 11 illustrating a part of a tire tread. The distribution of the auxiliary grooves 4 of a smaller width in the tread 1 is intended to impart an enhanced gripping property to the tire without an appreciable decrease in the actual treading area ratio of the tire.
Generally, the auxiliary grooves 4 are designed to furnish the tire with the gripping property without an appreciable sacrifice of the actual treading area ratio. It is, therefore, usual for the width of these auxiliary grooves 4 to be restrained in the range of 2 to 8 mm, i.e. 1 to 4% of the developed width D of the tread 1.
It is most desirable from the standpoint of traction that the depth of the auxiliary grooves 4 to be equal to that of lug grooves 3 constituting part of the main grooves. In the surface (tire tread) as viewed in its developed state, since the auxiliary grooves 4 are disposed preponderantly toward tread shoulders 2, an addition to the depth of the auxiliary grooves 4 tends to induce localized wear of the tread shoulders 2. The depth of the auxiliary grooves, therefore, is generally selected in design so as to fall in the range of 30 to 80% of the depth of the lug grooves 3 with due respect to the balance between the resistance to wear and the traction.
Incidentally, while the tire remains in contact with the ground, surface strain is concentrated on the bottom surface of the auxiliary grooves 4 because the tread is constantly exposed to compressive stress.
The tread shoulders 2 have a smaller radius from the rotary axle of the tire than the tread center 1a. On the assumption that the tread surface falls in one plane, the tread shoulders 2 are subject to greater displacement and the concentration of strain is proportionately large on the bottom surface of the auxiliary grooves 4 which are located toward the tread shoulders 2. Further, since the lug grooves 3 open into the tread shoulders 2, series of blocks arranged intermittently in the circumferential direction T of the tire constitute the tread shoulders. When the tire using the tread constructed as described above is set in place on the drive axle, therefore, the tread shoulders 2 are destined to sustain the surface strain due to the shear force which is exerted on the tread for transmission of the drive torque to the road surface.
In the bottom portions of the rib grooves M which continue generally along the circumferential direction T of the tire, the surface strain is generally in the form of compressive strain. The lug grooves 3 and the auxiliary grooves 4 which open into the lug grooves are mostly arranged in the direction of radius (namely the direction perpendicular to the circumferential direction T of the tire) owing to the requirement for traction and, therefore, are subject to repeated exertion of compressive strain and tensile strain. Owing to the repetitive exertion of these strains synergistically coupled with the high absolute levels of these strains, cracks occur preponderantly on the bottom surface of the lug grooves 3 disposed in the tread shoulders 2, with the deterioration (ozone cracks) of the tread rubber and the minute cuts as contributing causes.
In the pneumatic tire for a heavy duty truck which has a tread pattern illustrated in FIG. 11, the concentration of surface strain occurs preponderantly in opening parts 5 of the auxiliary grooves 4 in which lateral walls 6' of the lug grooves 3 opposed to the opening parts 5 of the auxiliary grooves 4 intersect the auxiliary grooves 4. Thus, cracks are most liable to originate in edge parts of the bottom surface of the auxiliary grooves 4. Further, since the portions sustaining such cracks are exposed to repeated exertion of strains during the travel of the vehicle, the growth of cracks proceeds inwardly in the tire wall and tends to result in total loss of serviceability or recapability of the tire.
Heretofore, alleviation of the surface strain and repression of the occurrence of cracks have been attained by smoothly rounding the corners of the edge parts 9 of the bottom surface of the auxiliary grooves in the openings 5 of the auxiliary grooves 4 where the lateral wall parts 6' and the auxiliary grooves 4 intersect each other as illustrated in FIG. 12 and FIG. 4 which depict cross sections taken through FIG. 11 along the line A--A. Unfortunately, this method has given no satisfactory solution to the problem because the concentration of strain still occurs around the edge parts 5 of openings of the auxiliary grooves. In FIG. 12 and FIG. 14, 7 denotes a bottom surface of the auxiliary grooves 4 and 8 a lateral wall part of the auxiliary grooves 4. FIG. 13 and FIG. 14 depict cross sections taken through FIG. 11 along the line B--B. FIG. 13 represents the case wherein the depth d of the auxiliary grooves 4 is large and FIG. 14 the case wherein the depth d is small. In FIG. 12, 3d denotes the depth of the lug grooves 3.
As another measure to cope with the problem, there has been tried a method for repressing the level of concentration of strain by either decreasing the depth of the auxiliary grooves 4 or approximating the proportions depth d and the width of the grooves to each other as much as possible as illustrated in FIG. 14. The decrease of the depth d of the auxiliary grooves 4, however, results in a drop of the traction during the early stage of wear. The increase of the width of the auxiliary grooves 4 brings about a decrease in the actual tread area ratio and inevitably entails a decrease of wear life.